import pygame
from pygame.sprite import Sprite

class Alien(Sprite):
    """表示单个外星人"""

    def __init__(self,ai_settings,screen):
        #初始化外星人及其位置
        super().__init__()
        self.screen = screen
        self.ai_settings = ai_settings

        #加载外星人图像，并初始化其位置
        self.image = pygame.image.load("images/enemy0.png")
        self.rect = self.image.get_rect()

        #每个外星人最初在屏幕左上角附近
        self.rect.x = self.rect.width
        self.rect.y = self.rect.height

        #存储外星人精确位置
        self.x = float(self.rect.x)

    def blitme(self):
        """在指定位置绘制外星人"""
        self.screen.blit(self.image,self.rect)

    def update(self):
        """向右移动外星人"""
        self.x += (self.ai_settings.alien_speed_factor * self.ai_settings.fleet_direction)
        self.rect.x = self.x 

    def check_edges(self):
        """如果外星人位于屏幕边缘，就返回True"""
        screen_rect = self.screen.get_rect()
        if self.rect.right >= screen_rect.right:
            return True
        elif self.rect.left < 0 :
            return True
        